The combustion of liquid fuels in a diesel engine typically leads to the formation of large amounts of nitrogen oxides and particulate soot emissions. It is desired to reduce these emissions.
Catalytic combustion technology has been applied to continuous combustion processes in which a fuel air mixture is continuously passed over a catalyst and all or a portion of this fuel is combusted in the catalyst. In a series of recent patents, U.S. Pat. No. 5,183,401 to Dalla Betta et al.; U.S. Pat. No. 5,232,357 to Dalla Betta et al.; U.S. Pat. No. 5,250,489 to Dalla Betta et al.; U.S. Pat. No. 5,281,128 to Dalla Betta et al.; and U.S. Pat. No. 5,425,632 to Tsurumi et al., catalytic combustion systems are described in which the fuel is partially combusted on a catalyst and the remaining fuel combusted downstream of the catalyst with little NOx formation since the fuel air mixture is very uniform and there are no peaks in temperature.
Unfortunately, such systems are not readily adaptable to diesel fuels (where NOx formation is very high) since in a diesel engine the fuel is a liquid with a rather high boiling point and such liquid fuel is injected directly into the cylinder when the piston is at the top of the compression stroke. Specifically, the diesel cycle depends on so called xe2x80x9ccompression ignitionxe2x80x9d (i.e.: the ignition of the fuel-air mixture upon injection of the fuel into the high pressure, hot air environment at the top of the compression stroke).
Moreover, emissions from a diesel engine result from the diesel cycle, (i.e.: from the direct injection of the diesel fuel into the hot, high pressure air in the cylinder). Some portion of the fuel vaporizes and the combustion occurs in the near stoichiometric fuel air mixture around the liquid droplet. As this fuel bums, it bums in this near stoichiometric region resulting in a very high flame temperature and high NOx. In addition, as the flame heats the liquid droplet, the droplet begins to pyrolyze and can thus form particulate matter that may not burn before the combustion products exit the engine. This also results in high levels of particulate emissions.
Modifications to diesel combustion process are thus desired to minimize the NOx and carbon particulate emissions. Unfortunately, various existing emission control processes have their own limitations.
For example, although engine modifications have helped to reduce emissions significantly, the future benefit from further engine modifications appears to be minimal. For instance, some of the approaches such as exhaust gas recirculation and staged or stratified combustion have not resulted in significant reduction in NOx. Moreover, trap and release technology treats the NOx emissions after they are formed. While trap and release approaches can reduce NOx, they are very costly and necessitate mechanically intensive systems that can significantly increase fuel consumption for the engine since the reductant is formed by either injecting raw fuel directly into the catalyst system to generate a reducing environment or by running the engine rich which is not optimal for a diesel engine.
A variety of existing emission control processes and their limitations are set forth below.
U.S. Pat. No. 5,307,772 describes a catalyst placed in the combustion chamber of a diesel engine, specifically in a prechamber or in the throat between the prechamber and the main cylinder chamber of an indirect injection engine or in the top of the piston of a direct injection engine. The prechamber of the indirect injection engine has a large volume and as described in the ""772 patent a significant amount of the combustion occurs in this prechamber. The catalyst described in the present invention may be much smaller than the catalyst of the ""772 patent and is placed between the fuel injector and the main combustion chamber in which the majority of combustion takes place. The fuel is injected through the catalyst with very little of the fuel combusting in the region between the injector and the catalyst. Some portion of the fuel is combusted within the catalyst and most of the fuel is combusted after or downstream of the catalyst in the main chamber. In the ""772 patent, the major amounts of NOx are said to be formed in the prechamber and then to pass through the catalyst on the way to the prechamber. According to this reference, this mixture is reducing and the catalyst is designed to reduce the NOx to N2. While this may reduce a portion of the NOx it would not impact the NOx formation during the remainder of the combustion process in the main part of the cylinder. In addition, the catalyst of the ""772 patent is described as being contained in a prechamber and prechamber engines have severe disadvantages, in particular, lower efficiency. The low efficiency is partly due to the pressure drop or flow restriction as the combustion gases flow from the prechamber to the main chamber. Lastly, the catalyst described in the ""772 patent consists of a ceramic monolith and is fully coated with catalyst on all surfaces, and all channels are continuous from inlet to outlet. The ""772 patent does not envision that a catalyst can be configured to impact the combustion process by vaporizing the fuel, nor does it envision a catalyst that could partially reform the fuel to produce components that would advantageously impact the combustion process. In addition, this patent does not envision that a catalyst could be designed that would limit the combustion temperature by using adjacent channels with no catalyst coating to act as heat exchange surfaces.
Another patent, GB 2 186 913 describes a catalyst coating on the internal surfaces of new engine design using two pistons acting in some concerted fashion. This catalyst is used only to initiate combustion. Numerous patents have also been issued which describe glow plugs for diesel engines with catalyst coatings or catalytic wires on the surface of the glow plug. See for example U.S. Pat. Nos. 4,345,555, 4,896,636, 5,146,881 and 5,593,607. In all cases, these patents describe a glow plug with increased performance. ascribed to the catalyst coating but do not describe any action of the catalyst to vaporize the fuel, further mix it with the air charge or to chemically process the fuel to more desirable components. In addition, none of these references describe a monolithic catalyst unit that is just downstream of the fuel injector and through which a major fraction of the fuel passes.
In a preferred aspect, the present invention comprises positioning a catalyst within the combustion chamber of diesel engine, preferably just downstream of the fuel injector.
The present invention offers the following advantages:
When fuel is injected into the diesel engine, the liquid fuel droplets impact the catalyst and will be partially oxidized with the release of heat assisting to vaporize the liquid droplets to form vapor phase fuel components. The resulting fuel air mixture that passes into the combustion chamber will therefore be at a high temperature and will result in a more uniform mixture of fuel and air. This more uniform mixture of fuel and air will combust with the formation of lower levels of nitrogen oxide emissions and particulate soot.
It is to be understood that the exact catalyst used in the present invention can take a number of different novel forms all of which are inventive within the scope of the present invention.
In contrast to the above examples, the present invention sets forth a system with the following characteristics: